A　ultrathin　HNb3O8　nanosheet　was　successfully　prepared　through　a　top-down　approach.　Atomic　force　microscopy　(AFM)　further　confirmed　that　the　thickness　of　the　nanosheet　was　about　1.30　nm.　The　structure　and　the　surface　chemical　state　of　the　HNb3O8　nanosheet　were　well-characterized　using　X-ray　diffraction　(XRD),　high-resolution　transmission　electron　microscopy　(HRTEM),　X-ray　absorption　fine　structure　(XAFS),　Raman　and　X-ray　photoelectron　spectroscopy　(XPS).　The　photocatalytic　hydrogen　evolution　activity　of　the　HNb3O8　nanosheet　was　about　4　times　higher　than　that　of　layered　HNb3O8　under　ultraviolet　irradiation.　The　enhanced　activity　was　ascribed　to　the　unique　two-dimensional　structure　with　a　molecular　thickness　that　leads　to　the　effective　separation　of　the　photogenerated　carriers.　Moreover,　a　considerable　variation　in　the　photocatalytic　hydrogen　evolution　activity　of　the　HNb3O8　nanosheet　was　observed　when　suitable　metals　were　loaded　on　the　nanosheet　via　in　situ　photodeposition.　©　The　Royal　Society　of　Chemistry.